The present invention is directed to an ultrasonic method for quantifying the amount of low density deposit on the inner surface of boiler waterwall tubes.
Deposits in waterwall tubing can be characterized according to density and chemical composition. Such deposits are generally of the same chemical makeup which is magnetite (Fe.sub.3 O.sub.4), however, their densities vary considerably due to differences in porosity levels. Presently, a service referred to as NOTIS.TM. is applied to "high" density deposits in steam-filled tubes (e.g. reheater or superheater tubes). Steamside oxide deposits have a density of greater than 90% of the theoretical density of magnetite which is 5.18 gm/cc. The amount of porosity contained in steamside scale is low, therefore an ultrasonic pulse can travel through this medium with little resistance. U.S. Pat. No. 4,669,310 disclosed the NOTIS.TM. service which uses ultrasonic analysis.
In the case of "low" density deposits or waterside oxide, which grows in furnace waterwalls, the porosity level is high. Waterside deposit density is 50% (.+-.20%) of magnetite's theoretical density. With this amount of porosity, an ultrasonic pulse has difficulty finding a continuous medium for travel. As the pulse collides with the individual pores, pulse energy is dissipated, decreasing its capability to travel further into the deposit. The greater the porosity, the shorter the travel.
The NOTIS.TM. service and U.S. Pat. No. 4,669,310 measures "high" density deposits and involve determining the pulse travel time between individual signals thus generating a quantitative result, e.g. in mils of oxide.
U.S. Pat. No. 4,685,334 relates to the ultrasonic detection of hydrogen damage in boiler tubes. This reference teaches the use of angle-beam ultrasonic shear-waves introduced via a pitch-catch technique in either the axial or the circumferential directions of a boiler tube to detect the presence of hydrogen damage. The differences in relative attenuation in scanning occurring in an undamaged area and a hydrogen damaged area are used to detect hydrogen damage.
U.S. Pat. No. 3,901,071 is directed to an ultrasonic thickness gauge having ultrasonic probes positioned around the circumferences of a pipe to monitor the pipe thickness by the time interval between echoes from the outer and inner surfaces of the pipe.
U.S. Pat. No. 4,446,736 describes an ultrasonic method for testing the integrity of the internal lining of a hollow body. This reference teaches that a lining that is intact will absorb the ultrasonic wave and will not reflect the wave. Accordingly, a reflected wave frequently indicates a loss of the lining on the adjacent wall. Comparison of the reflected wave with predetermined standards provides an indication on whether or not the lining is intact.
In addition, the following technical references are pertinent to the present invention:
1. Gordon, Jr., B. E., Measurement of Applied and Residual Stresses Using an Ultrasonic Instrumentation System, ISA Transactions, Vol. 19, No. 2, pages 33-40, presented at the ISA Symposium 1978, Albuquerque. PA1 2. Mitigating Forced Outages By Selective Replacement of Boiler Tubes, Loper, Schoemaker, and Stromp, Technical Article presented at EPRI in Bel Harbor, Fla., Apr. 13-15, 1983. PA1 3. Ultrasonic Detection of Calcium Sulfate Scale on Metal Surfaces, by Fred R. Rollins, Jr., U.S. Department of Interior, Office of Saline Water, Research and Development Progress Report No. 444.
Technical reference 1 relates to an ultrasonic inspection system developed for nondestructive measurement of applied and residual stresses. The system measures time of flight of an ultrasonic pulse through a material.
Technical reference 2 discloses a method of inspecting boiler tubes for hydrogen damage. This reference teaches employing a 5 MHz transducer with a contact straight beam and calibrating to ensure a total of four back reflections. The reference further states that accurate thickness calibration identifies thinning as well as identifying hydrogen damage. The procedure is based on an attenuation principle.
Technical reference 3 shows a series of oscilloscope traces taken with an ultrasonic frequency of about 9.8 MHz used on water filled tubes. In this particular case, the deposit was of intermediate density and the influence on the ultrasonic pattern was about an average between scale conditions that produced both stronger and weaker effects. The reference states that if physical properties of the scale, such as density, particular size, etc. remain reasonably constant then the decay rate of the echo train should correlate quite well with the thickness of the deposit. The tubes must be water-filled for this method to operate, however.